One of the hallmarks of Alzheimer's disease (AD) or senile dementia of Alzheimer type (characterized by the disturbance of memory) is the presence of characteristic lesions called senile plaques, which are observed from the earliest stage of the disease in the brains of Alzheimer patients. Senile plaques contain aggregates of amyloid β protein (amyloid beta-protein; Aβ) a protein having a 40-amino acid and a 42-amino acid form. The formation and accumulation of senile plaques is highly specific to Alzheimer's disease and precedes the neurofibrillary degeneration that is also characteristic of the disease. Thus, the plaques are assumed to be a leading cause of Alzheimer's disease. The accumulation of Aβ has also been observed in the brains of patients with Down's syndrome. Thus, the accumulation of Aβ has also been believed to be a major cause of the neurological symptoms of Down's syndrome. Because of this, suppressing the accumulation of Aβ has become of major interest as a therapeutic strategy for neurodegenerative diseases.
A recent report describes a method for suppressing the accumulation of Aβ using Aβ vaccination in a mouse model (Schenk et al., Nature 400:173–177, 1999). In this study, the authors aim at preventing the deposition of Aβ in the brain by inducing, via vaccination, a host immune response to Aβ.
In another report, transgenic mice overexpressing a gene encoding a mutated amyloid precursor protein (APP) showed deposition of Aβ in the brain with aging. However, immunization of the mice with Aβ1-42 at an early age (before the onset of Alzheimer-type neuropathologies) induced the production of antibodies against Aβ and prevented the development of Aβ-plaque formation. Immunization of older mice markedly decreased Aβ deposition in the brain and reduced the extent and progression of AD-like neuropathologies (Schenk et al., Nature 400:173–177, 1999). This suggests the possibility that antibodies produced in the body bind to Aβ and inhibit the formation of amyloid aggregates. The hypothesis is supported by the documented result that the direct peripheral administration of an anti-Aβ antibody also suppressed Aβ deposition in the brain (Bard et al., Nature Med. 6:916–919, 2000).
In addition, other reports describe the anti-AD effect of Aβ vaccines. For example, the inoculation of Aβ not only reduced the deposition of Aβ protein but also improved the impaired learning ability of APP mutant mice with aging (Janus et al., Nature 408:979–982, 2000; Morgan et al., Nature 408:982–985, 2000).
These findings provide supporting evidence for the idea that immunotherapy using Aβ protein as a target is useful as a method for treating AD and the like. However, Aβ protein is present in the serum as well as in AD brain lesions. Accordingly, an anti-Aβ antibody administered to a patient may react to Aβ protein in serum, forming immune complexes. Such immune complexes may potentially lead to kidney disorders. In other words, kidney disorders may be potential side effects of immunotherapy using Aβ protein as a target.
There are two types of Aβ proteins, Aβ42 and Aβ40. Aβ42 is a 42-amino acid protein whose sequence is DAEFRHDSGYEVHHQKLVFFAEDVGSNKGAIIGLMVGGVVIA (SEQ ID NO:1). Aβ40 is a truncated Aβ protein lacking the last two amino acid residues of Aβ42 at the C-terminal end. Truncation is achieved by digestion with secretase γ. Aβ42 is inclined to aggregate more easily than Aβ40. Thus, Aβ42 is assumed to play an important role in the accumulation of Aβ.
AβN3pE-42 is a particular species of Aβ42 which lacks the first two amino acid residues at the N-terminus of Aβ42 and has a pyroglutamate which has been converted from the glutamic acid (E) at the third amino acid position. AβN3pE-42 is a major protein constituent of senile plaque, and accumulates from early stages of Down's syndrome and Alzheimer's disease (Saido et al., Neuron 14: 457–466, 1995; Harigaya et al. Biochem. Biophys. Res. Com. 276: 422–427, 2000). A glutamic acid (E) is pyroglutamylated to a pyroglutamate residue (pyroglutamyl formation). Recent studies have showed that AβN3pE-42 comprises nearly 50% of Aβ protein accumulating in brain tissues. The amino acid sequence containing a pyroglutamylated glutamic acid residue at the third amino acid position exhibits a longer half-life in the brain. Thus, the protein containing the pyroglutamylated amino acid is presumed to remain in the brain for a long period. These facts indicate that ensuring the removal of AβN3pE-42 is a therapeutically important challenge.